The present invention relates generally to an improved tamper-resistant, reciprocating cylinder lock and lock bolt keeper mechanism. The apparatus of the present invention provides structure for reducing the likelihood of dislodgement of the bolt from a locked position when the lock mechanism is under physical attack.
Conventional, non-reciprocating cylinder locks are typically fixedly mounted to a solid surface, secured against any movement other than rotational movement of the lock cylinder upon insertion of the proper key (e.g., a door lock mounted in a mortise). Reciprocating cylinder locks, in contrast, are designed to permit longitudinal movement of the entire lock mechanism as well as key-actuated rotational movement of the lock cylinder. Reciprocating cylinder locks have a variety of common uses, including vending machines, bicycle shackles, and automobile security devices, for example.
A reciprocating cylinder lock is conventionally mounted within a casing having inner and outer ends, the form of which is determined by the particular use of the lock. A longitudinal passage in the casing defines a cylindrical lock chamber and permits insertion of a protective tubular sleeve inside the chamber. The tubular sleeve has inner and outer ends and a cavity or opening therein extending transverse to the axis of the sleeve. The sleeve is mounted for axial movement between a retracted locked position and an extended unlocked position, and the sleeve is normally urged toward its extended position. A bolt is mounted for movement through the sleeve cavity along a linear path transverse to the axis of the sleeve between a retracted, unlocked position and an extended, locked position. The bolt is normally urged toward the extended, locked position.
Also part of the lock mechanism are a bolt keeper for engaging the bolt when the bolt is in its extended position and a rotatable locking shaft within the sleeve having inner and outer ends. The locking shaft includes structure responsive to rotation of the shaft for retracting the bolt from its extended, locked position. The outer ends of both sleeve and lock shaft are designed for insertion of a key.
In an unlocked condition, a portion of the sleeve-encased lock shaft will extend longitudinally outward from the outer end of the casing. The reciprocating cylinder lock can be moved to a locked position by depressing the longitudinally-extending portion of the sleeve-encased shaft until it is flush with the outer end of the casing. This inward axial movement of the lock shaft aligns the spring-biased bolt with a casing cavity adjacent to the keeper. In this position the bolt spring urges the bolt toward the casing cavity, and brings the bolt into engagement with the lock bolt keeper. A major advantage of such reciprocating devices is that no key is required to place the device in a locked position.
It has been found that conventional reciprocating cylinder locks, particularly those in which the spring-biased bolt extends vertically upward from the lock mechanism, can be defeated by physical attack on the lock mechanism itself. Repeated hammering on the lock with any implement which can convey a force can cooperate with the normal force of gravity to dislodge the bolt from engagement with the bolt keeper and force the bolt more fully into the sleeve cavity, which permits the sleeve to be returned to its unlocked position. In a like manner, tampering can also be effective in dislodging reciprocating cylinder locks in which the bolt is not in an upright position, depending on the strength with which the lock is repeatedly hit with an instrument, such as a hammer.
An example of a reciprocating cylinder lock which would be susceptible to such tampering, is described in Scherbing, U.S. Pat. No. 4,083,211. A reciprocating cylinder lock is at particular risk when incorporated into a mechanism such as an anti-theft automobile security device. This is because of the frequency with which auto thefts are attempted and the persistency with which a car thief will tamper with an individual anti-theft device.
There exists, therefore, a need in the art of locking mechanisms for a reciprocating cylinder lock which is resistant to dislodgement by common forms of physical tampering, such as hammering.